Payback period investigation of the organic Rankine cycle with mixed working fluids to recover waste heat from the exhaust gas of a large marine diesel engine

The aim of this study is to investigate the payback period and the payback-period reduction of the organic Rankine cycle (ORC) with mixtures for waste heat recovery from the exhaust gas of a large marine diesel engine. Working fluids with zero-ozone-depletion potential, such as R236fa, R245fa, 8600, R1234ze, and their mixtures, are selected to analyze the economic and thermodynamic performances in the conversion cycle. The optimal mass fractions of the mixed working fluids R236fa/R245fa, R236fa/R1234ze, and R600/R1234ze for the lowest payback period with the corresponding optimal expander inlet temperature and condenser outlet temperature of the ORC have been obtained. R600/R1234ze possesses the best performance in terms of payback period. Under optimal conditions, the payback period of the ORC with R600/R1234ze is the lowest and is shorter than that with R236fa, R245fa, MOO, R1234ze, R236fa/R245fa, and R236fa/R1234ze by 7.55%, 6.47%, 9%, 9.17%, 0.9%, and 2.88%, respectively. To evaluate the economic potential of mixed working fluids using different types of expander such as turbine, scroll, and screw expanders properly, the relationships between the isentropic efficiencies of the expander and their corresponding optimal expander inlet temperature and optimal mass fraction are obtained. Optimal correlations are proposed to conveniently predict the optimal mass fractions of the mixtures R236fa/R245fa, R236fa/R1234ze, and R600/R1234ze in the ORC and to assess the reduction of their payback periods for the waste heat recovery of a large marine diesel engine. The results highlight the significant potential of the mixtures used in the ORC for performance improvement and demonstrate the importance of mixture assessment for reducing the payback period in further work.